The US and Russia say they want a new nuclear arms treaty to enter force at soon as possible, after failing to agree a successor to the Start I pact.

The nations uphold the "spirit" of the 1991 Cold War-era treaty despite its end, the US and Russian presidents said in a joint statement.

Talks on a new accord are expected to continue after the treaty expires. at midnight on Friday.

Russia's foreign ministry said "intense efforts" were ongoing on a new treaty.

US President Barack Obama and his Russian counterpart, Dmitry Medvedev, agreed in July that a new treaty should bring deep cuts in nuclear warheads.

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